The ribonucleoside triphosphate reductase (RTPR) from Lactobacillus leichmannii has previously been shown to catalyze the homolytic cleavage of the carbon-cobalt bond of AdoCbl, and the resulting paramagnetic species has been characterized by rapid freeze-quench EPR spectroscopy (OrmeJohnson, 1974). Simulations of X-and Q-band EPR spectra modeling this radical intermediate as a thiyl radical coupled to cob(II)alamin by electron-electron exchange and dipolar interactions yield reasonable fits to spectra obtained at both microwave frequencies (Gerfen, 1997). Simulations that employ either a single-spin model or one with cob(II)alamin coupled to an organic radical do not. This provides support for a thiyl radical intermediate in this system.